Method for locating mobile terminals, system and components therefor

ABSTRACT

To locate a mobile terminal (MS 1,  MS 2 , . . . ) within a mobile communication network comprising at least a radio base station (BTS 1,  BTS 2,  . . . , BTSn), a set of physical dimensions are measured, which identify, according to respective functions, the location coordinates (x, y, z) of the mobile terminal. The method comprises the steps of: generating, starting from said set of physical dimensions and respective functions, a global locating error function (φ) which has a minimum for values of said locating co-ordinates (x, y, z) corresponding with the position occupied by said mobile terminal, seeking the minimum of said error function (φ) by varying at least one of said locating co-ordinates (x, y, z), and locating said mobile terminal in correspondence with the value of said at least one locating co-ordinate corresponding to said minimum.

TECHNICAL FIELD

The present invention relates to the geographic location of mobileterminals within telecommunication networks.

BACKGROUND ART

In the prior art, various solutions are already known in which theterminals belonging to a mobile radio network are located on the basisof the physical signals and of the information available in the network,that is, without the aid of external equipments or systems, such assatellite positioning systems.

The various known solutions, aimed at allowing to locate mobileterminals on the basis of the physical signals and of the informationavailable in the network, are distinguished from each other by thecombination of two key aspects: the type of data provided to theposition calculation system and the processing methodology applied tosaid data.

In regard to the first aspect, there are at least four types of physicalquantities that can be measured by the mobile radio network and/or bythe mobile terminal in order to obtain information useful for locatingpurposes.

In the first place, it is possible to measure the power received by themobile terminal from a certain radio base station (currently indicatedas BTS in the case of GSM and similar systems). This solution allows toobtain a circumference—centred on the base station underconsideration—whereon, in an unknown point, lies the mobile terminal.Combining multiple power measurements and then intersecting therespective circumferences, the point where the mobile terminal lies canbe identified. Power measurements are intrinsically inaccurate, becausethey are influenced by many uncontrollable factors, the most significantof which are antenna gain and fading, a phenomenon involvingelectromagnetic wave propagation, which cause the random andunpredictable fluctuation of the signal level.

It is also possible to measure the Timing Advance (TA), i.e. the “timeof flight” of a reference signal such as a control burst between thebase station and the mobile terminal (downlink) and, symmetrically,between the mobile terminal and the base station (uplink). Therefore,the value of TA indicates the distance between the mobile terminal and abase station. Like power measurements, TA also identifies acircumference whereon the mobile to be located lies. Combining multipleTA measurements (and then intersecting the corresponding circumferences)it is possible to identify the point where the mobile terminal lies. Inthe case of GSM and GPRS networks, TA measurements are inaccurate, bothbecause of the ways by which they are taken, and because of thequantization error due to the finite number of bits used to store themeasurement in the radio base station: in practice, the measure of TAallows to identify annuli with radial extension of about 550 m.

There are also the measurements of Observed Time Differences (OTD),obtained by measuring the difference between the distance from a mobileterminal and a base station and the distance from the same mobileterminal and an another base station. The OTD measurements describehyperbolas that, appropriately combined, allow to locate the mobileterminal. The OTD measurements provide results that are intrinsicallymore precise than the two described above, because they are based on themeasurement of the difference of the “times of flight” of anelectromagnetic field (as evidenced by the fact that the GPS system,universally known as the most accurate positioning system currentlyavailable, is based on the same type of measurements).

Lastly, there are the measurements of Time of Arrival (TOA), entirelysimilar to the OTD measurements with the difference given by the factthat the measurement is taken by the network and not by the mobileterminal.

Both OTD measurements and TOA measurements have the drawback derived bythe fact that, to yield accurate result, they require an exactsynchronisation between the base stations: this condition requires to beachieved the presence, within the network, of additional synchronisationdevices.

The four types of measurements described above are used to calculate theposition of a mobile terminal both operating in an absolute way, i.e.intersecting the geometric loci described by the measurements taken, andcomparing the available measurements with maps prepared a priori.

In the prior art there are different systems based both on the firstmethod (power) and on the second method (TA), which are furtherdifferentiated by the type of measurements whereon the locatingoperation is based.

For instance, in U.S. Pat. No. 5,613,205 the position of a mobileterminal is estimated by intersecting the geometric loci derived fromthe combination of OTD and power measurements.

In WO-A-0018148 and U.S. Pat. No. 6,167,274, in order to locate a mobileterminal, the measurements of the power received by the mobile from acertain number of base stations are compared with a database whichcontains the power “signatures” of a certain area as a function of thegeographic co-ordinates.

However, locating systems currently available in the art leave threefundamental issues unresolved.

In the first place, it is not considered that real locating scenariosare affected by measurement errors of various kinds, which have aconsiderable impact on locating accuracy (solutions like the onedescribed in the document U.S. Pat. No. 5,613,205 in fact consider onlysome of the more relevant errors). Among the main errors to beconsidered are those made in geo-referencing the base station (typicallyin the order of a few tens of metres with peaks in the order of hundredsof metres), in measuring the times OTD and TOA due to the lack ofsynchronisation of the base stations (typically with geometricequivalents in the order of tens of metres), in measuring the powerreceived by a mobile due to antenna gains and fading and, lastly, inmeasuring all mentioned parameters due to the systematic and intrinsicerrors of the measurements themselves and to the multipath of thephysical signals.

As a consequence of these errors, known locating systems yield pooraccuracy. Moreover, for the methods based on the intersection ofgeometric loci, the various measurements can also diverge, preventingthe estimation of the mobile terminal position: in fact, due to themovement of the geometric loci caused by the errors, there can be eitherno intersection or more than one. In the second place, for the methodsthat are based on the comparison between the signals received by theterminal and a database of “geographic signatures” of the signals, it isnecessary to constantly update the database as the mobile radio networkevolves. This updating operation is far from simple and the common riskis to compare the received signals with an obsolete database. Moreover,for practical reasons, the database is built using data calculated withmathematical models. Even in the best cases, this entails a differencerelative to the values measured by the terminal in the field, and isanother source of errors (see for instance U.S. Pat. No. 6,167,274).Lastly, the methods presented in the literature and commonly known arenot able to combine all types of measurements (power, TA, OTD and TOA)in a flexible way, but are limited at most to combine them in a rigidfashion, for instance, OTD measurements and power measurements (see U.S.Pat. No. 5,613,205). Consequently, when the prescribed measurements arenot available, the locating system is incapable of adapting itself tothe real measurement scenario, which it has to deal with, and thereforeit is not able to perform its functions.

DESCRIPTION OF THE INVENTION

The present invention is aimed at providing a solution able to overcomethe drawbacks described above.

According to the present invention, said aim is achieved with a methodhaving the characteristics specifically listed in the claims thatfollow.

In particular, the invention relates to a method for identifying theposition of mobile terminals: on the basis of a plurality of signals orphysical quantities, corresponding error functions are determined whichallow to calculate a global error function having a minimum incorrespondence with the position of the mobile terminal to be located.

The invention also relates to the corresponding system and theassociated components.

Among said components it is also included a software product able to beloaded directly in the memory of a digital computer (as is the case ofcurrently produced mobile telephones) associated with a mobile terminalfor telecommunication networks. The software product under investigationcomprises portions of software code that can implement at least a partof said integrated locating module, according to the invention, in themobile terminal itself when the software product is run on said digitalcomputer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by the means of a non-limitingexample and with reference to the accompanying drawings, in which:

FIG. 1 shows, in the form of a functional block diagram, theorganisation of the system according to the invention, and

FIG. 2 is a flow chart illustrating the implementation of the methodaccording to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The diagram of FIG. 1 shows a preferred embodiment of the invention,with reference to the interaction between a “communication” or “network”environment, designated as CA, and a “locating” environment: the lattercan be seen essentially as an implementation and development of a knownlocating function (Mobile Location Centre or MLC) already present in thenetwork/system.

The CA communication environment essentially corresponds to a normalmobile radio network operating according to any GSM, GPRS, UMTS orequivalent standards, including new generation developments. Thesolution according to the invention is therefore “transparent” regardingthe characteristic specifications of the CA communication environment.

The network under investigation thus comprises n (n>0) base stations(indicated for the sake of simplicity as BTS1, BTS2, . . . ) as well asone or more mobile terminals MS1, MS2 . . .

The general operating criteria of such a network, whichever standard isadopted (GSM, GPRS, UMTS or others) are wholly known in the art andhence do not require a detailed description herein.

The locating environment MLC has the purpose of determining the positionof mobile terminals MS1, MS2 . . . in terms of location co-ordinates (x,y) within the territory covered by the communication network.

The MLC environment mainly comprises:

-   -   a supervision module SM overseeing all the operations of the        locating system;    -   an accounting and billing module AB, and    -   a gateway module GW destined to act (according to criteria        better described below) as an interface towards an IP network        whereto are connected end users and/or service providers        globally designated as U.

Within the locating environment MLC the following functions areprovided:

-   -   a position calculation function PCF, and    -   a communication management function.

The communication management function is normally performed by dedicatedmodules one of which, indicated as MGC, resides at the fixed networklevel, for instance at a network management node. Each of the mobileterminals MS1, MS2 . . . is also provided—in a manner known initself—with a corresponding communication management module, notexplicitly illustrated in the drawings.

In an additional embodiment, the terminals can also be provided withoptional devices to conduct measurements to complement those carried outby the network. For instance, the terminals can comprise an altimetry,pressure measuring devices, devices able to measure distances, etc.,such as to allow to identify, with a determined level of precision, atleast one geographic co-ordinate or a distance from a predeterminedgeographic co-ordinate. The availability of such devices allows toprovide the PCF function with additional information able to enhancelocating precision, as shall be described in detail below.

The aforesaid optional devices can also be installed in the BTS, as isreadily apparent to a person versed in the art.

If the measurements do not vary over time, for instance, in a determinedarea, such measurements can be inserted, in the form of appropriatestatic values or functions for their calculation, in the SM module sothat, through the GW module, they are transferred to the PCF function.

The position calculation function can instead reside at the networklevel (as shown in continuous lines in the accompanying drawings), atthe level of the mobile terminals MS1, MS2 . . . (as indicated withdashed lines in the accompanying drawings), and also at both levels.Therefore, the specific solution selected is dictated by system designconsiderations (processing power available in the various locations, etc. . . ) and in fact it has no bearing on the understanding of theinvention.

The currently preferred embodiment of the invention provides twopossible operating modes, i.e.

-   -   locating operation invoked by the individual mobile terminal        involved    -   locating operation invoked by the supervision SM module.

If one of the mobile terminals MS1, MS2 . . . invokes the locatingoperation, the determination of its position entails the completion ofthe following operations:

-   -   the mobile terminal involved MS1, MS2 . . . selects the set of        data whereon the position is to be calculated (dynamic or        pre-set selection); if said set includes the TOA data, the        mobile terminal must require from the network said values which        are provided for instance via SMS or an ad-hoc data        communication protocol;    -   the mobile terminal measures the values of the set defined        above;    -   the mobile terminal requests from the network the geographic        positions of the base stations whereto the above measurements        refer, and the network communicates them to the mobile terminal,        for instance by means of broadcast or SMS;    -   the PCF function in the mobile terminal computes the position of        the terminal itself;    -   only if required, the position just calculated is sent (for        instance via SMS or ad hoc data communication protocol) to the        supervision module SM in view of possible additional processing        in order to provide additional services (for instance tracking,        tourist guide, transmission of information that depends on        geographic position, etc.), such data being obtained for        instance from providers U through the IP network.

If the supervision module SM invokes the locating of a determined mobileterminal, the following operations are carried out:

-   -   the set of data on which the position is to be calculated is        selected. If said set includes the OTD data, the supervision        module SM must request the involved mobile terminal to provide        said values that are provided for instance via SMS or ad-hoc        data communication protocol;    -   the values of the set considered above are measured;    -   the PCF function at the MLC level calculates the position of the        mobile terminal;    -   only if requested, the position just calculated is sent (for        instance via SMS or ad hoc data communication protocol) to the        mobile terminal usually together with the other information        (tourist guide, transmission of information which depends on the        geographic position, road map of the area, etc.) already seen        above.

Once again it is stressed that the solution according to the inventionis applicable to any mobile radio network (GSM, GPRS, UMTS or others)that complies with the related international specifications andcomprises a certain number of radio base stations, mutually connected bymeans of a core network, and a certain number of mobile radio terminalswhose geographic position (for instance latitude and longitude) is to beestimated.

The core of the locating system illustrated herein is constituted by thePCF function, destined to calculate the position of the mobile terminal.

For this purpose, the PCF function, regardless of its location, receivesat its input the data needed to calculate the position (level of powerreceived by the terminal, TA, OTD, TOA or a combination, evenincomplete, thereof) and provides at its output the unknown position ofthe mobile. Since some data among those mentioned can be measured eitherby the network alone (TOA) or by the mobile terminal alone (OTD),communication protocols are used (implemented by the MGC communicationmanagement function) which transfer them from the mobile terminal to thenetwork and vice versa.

In particular, if the PCF function resides on the mobile terminal and itis necessary to use the TOA data because the other data are notavailable, the supervision module SM (the only one that knows the TOAdata) makes such data available to the mobile terminal.

In wholly similar fashion, if the PCF function resides on the network,the mobile terminal involved in the locating action makes available tothe network, if this is necessary, the data which the network cannotmeasure (e.g. OTD).

All this while the gateway GW (preferably constructed according to thestandard ETSI TS 101.724 V.7.3.0 (2000-02)—“Digital cellulartelecommunications system (Phase 2+); Location Services (LCS);(Functional description)—Stage 2; (GSM 03.71 version 7.3.0 Release1998)” manages the information security and the routing of theinformation between IP network and the locating system.

Regardless of where it is physically located, the PCF function forcalculating the position operates by minimising a combination of errorfunctions defined in the manner described below, where (x,y) are the(unknown) co-ordinates of the terminal, n is the number of availablebase stations and dist calculates the absolute distance between twopoints:

-   -   for TA measurements:        f _(j)(x,y)=dist(MS,BTS _(j))−TA _(j) j=1, . . . , n    -   for OTD measurements:        f _(k)(x,y)=dist(MS,BTS _(i))−dist(MS,BTS _(j))−OTD _(i,j)        i,j=1, . . . , n; k=1, . . . , n!/2

It will be appreciated that the above also applies, respectively:

-   -   to power measurements, because, similarly to what is defined for        TA measurements, they are circumferences centred on the        reference base station    -   to TOA measurements, because they are simply measurements        similar to the OTD measurements, but carried out by the network        instead of the terminal    -   to other measurements, for instance measurements of altitude        over mean sea level, obtained by means of optional devices; in        this case, they are curves that depend on the type of physical        dimensions measured and that, for instance, in the case of        heights over mean sea level, can be represented as a quadratic        function proportional to the altitude difference between what is        indicated by the device, for instance an altimeter, and the        actual altitude of the mobile terminal, in the following form:        f _(h)(x,y,z)=(z−z _(aitimeter))²        in which Z_(aitimeter) is the altitude indicated by the        altimeter.

For other types of measurements, as will be readily apparent to thoseversed in the art, distance functions corresponding to the measurementscarried out can be used.

It will be appreciated that any optional measurements as mentioned aboveallow the method to carry out the locating operation, improving both itscalculation times and its precision.

Consider, for instance, the additional measurement of the altitude ofthe terminal MS above the mean sea level. In this case the locatingmethod is very efficient because the altitude measurement limits thefield of existence of the solution itself to a determined iso-altimetriccurve or to a set of values proximate to the identified measurement.

The locating method can thus derive considerable advantages therefrom,in terms of velocity of convergence of the PCF function in seeking theminimum value, at least with respect to the times to be expected if allco-ordinates were unknown. In fact, to the function f_(h), asexemplified, can be given a high weight in the global error function(since f_(h) is based, for instance, on z_(altimeter) which is a veryprecise measurement) thereby allowing the iterative process to reach theexact altitude in just a few steps and, just as fast, to converge withthe other two co-ordinates, for instance x and y, which, as will bereadily apparent to a person versed in the art, are forced to movearound a curvilinear abscissa.

Moreover, knowledge of the altitude of the terminal MS also allows toimprove locating precision, because this knowledge provides more preciseinformation than the other available measurements (OTD, TA, . . . ) andsuch as to constrain the solution in an area around the exact point.

It will be appreciated that both for the terms TA_(j) and for the termsOTD_(ij) (or equivalent for power and TOA measurements) in generalexpressions of the following type applyTA _(j) =ta _(i) ·c±ε _(taj)OTD _(ij) =otd _(ij) ·c±ε _(otdj)where c indicates the speed of light in a vacuum, the term in lower caseletters expresses the “exact” value of the measurement and the term eexpresses the error component.

All measurements conducted and available are sent to the PCF functionwhere they are combined in a global error function (or, rather,functional) Φ(ƒ_(i)) whose minimum is sought with the variation of thex, y co-ordinates of the mobile terminal.

In particular, considering m available functions—corresponding to atotal of m measurements of power and/or OTD and/or TA and/or TOA—the PCFfunction operates seeking the minimum value of Φ${{\min\limits_{x,y}{{\Phi\left( f_{i} \right)}\quad i}} = 1},\ldots\quad,m$where Φ(ƒ_(i)) can be, for example, Φ=Σf_(i) ² or Φ=var(ƒ_(i),0) withi=1, . . . , m or yet other functions which minimise total errorcontribution and where m depends on the number of basic measurementsavailable.

The function—is thus continuous in the plane x, y and has a localminimum in which the total error contribution of all functions f_(i)with i=1, . . . , m is minimum.

The minimum can be equal to 0 (zero) only if the measurement errors ofphysical dimensions such as power, TA, OTD, TOA etc. are null.

In general, this situation is impossible in real cases.

The x, y co-ordinates in which the global error function Φ is minimum,correspond, according to the present invention, to the positionco-ordinates of the mobile terminal with maximum probability.

As an example of application of the criterion described above, considerthe case in which the mobile terminal involved in the locating actionmeasures the TA relative to the serving radio base station and alsomeasures the OTD relative to another radio base station. In this case,there are two available error functions:ƒ₁(x,y)={square root}{square root over ((x−X ₁)²+(y−Y ₁)²)}−TA₁ƒ₂(x,y)={square root}{square root over ((x−X ₂)²+(y−Y ₂)²)}−{squareroot}{square root over ((x−X ₁)²+(y−Y ₁)²)}OTD₁₂where (x,y) are the unknown co-ordinates of the mobile terminal and(X₁,Y₁) and (X₂,Y₂) are, respectively, the co-ordinates of the first andof the second base station.

The position of the mobile terminal can therefore be calculated by thePCF function$\left( {x,y} \right) = {\min\limits_{x,y}\left\{ {{var}\left( {{f_{1}\left( {x,y} \right)},{f_{2}\left( {x,y} \right)},0} \right)} \right\}}$where “var” indicates variation.

The position thus found is not affected by the geo-referencing errors ofthe base stations (errors always present in real mobile radio networks),by the synchronisation errors of the base stations themselves and by theerrors in measurement of the various reference dimensions (powers, TA,OTD and TOA).

The solution described performs an operation of minimisation of saiderrors and, naturally, if the errors mentioned above were null, eachfunction ƒ_(j) would have a zero in the position occupied by the mobileterminal.

In the presence of the aforesaid errors, the function reaches in anycase a minimum (and not a zero) in the point where it is most plausiblethat the mobile is located because the total error contribution isminimal.

In other words, the solution according to the invention is not limitedto seeking the intersection of geometric regions (hyperbolas,circumferences, etc.), which in the presence of the above errors couldnot exist, but derives the point in which the mobile terminal is mostplausibly located, thereby compensating for the various errors.

Calculation of the minimum can take place with various methods, forexample with Newton's method, which is well known in mathematics andquite proven. All methods share the fact that the search for the minimumalways converges to a solution and that this solution is the result ofan iterative process that starts from a point (x₀, y₀) and that, movingin the x,y plane on a succession of points (x₁, y₁), . . . , (x_(n),y_(n)) converges to the point in which the function has a relativeminimum. The iterative process stops in a point (x_(n), y_(n)) when theabsolute distance between the point itself and the previous one(x_(n-1), y_(n-1)) is less than a certain threshold, for instance 10 m.

The solution described herein is extremely flexible because it isapplicable when even a single radio base station is available.

It will be appreciated that even the—precise—determination of thelocation of a mobile terminal on a circumference centred around a basestation constitutes a location, both in itself (insofar as it issufficient to know which distance separates the mobile terminal frombase station), and in that it can be combined to other mechanisms orinformation able to identify the position of the mobile terminal on adetermined portion of the circumference.

The solution described herein is applicable to any type of measurementand to any combination of the available measurements, as it adapts oneach occasion to the contingent situation of the measurement scenario.

In particular, the solution described herein is applicable in athree-dimensional reference system, for instance using measurements ableto determine the height of the MS terminal above mean sea level.

In fact, in the case of a three-dimensional reference system, it issufficient to express the PCF function for seeking a minimum value inco-ordinates x, y, z instead of in co-ordinates x, y without changinganything in the method described.

In this case, the iterative process will start from a point x₀, y₀, z₀to converge to a solution x_(n), y_(n), z_(n), when the absolutedistance between the point itself and the previous one x_(n-1), y_(n-1),Z_(n-1) is smaller than a determined threshold, for example 10 m.

In reference to the flow chart of FIG. 2, from the viewpoint of thelocating system in the example described above, the following actions,starting from an initial step, designated as 100, are accomplished:

-   -   the mobile terminal (or the SM supervision module, possibly upon        command from an end user or a service provider U, through the IP        network) invokes the locating operation (step 102);    -   the supervision module SM verifies through the AB module that        the user that requested the locating operation is enabled for        the service and requests the mobile terminal to provide the        measurements whereon the locating operation is to be performed        (step 104);    -   supposing that, based on the choice made by the SM module,        either the or one of the PCFs residing at the network level has        to calculate the position of the mobile terminal (and not the or        one of the similar functions residing on the mobile terminal),        the mobile terminal collects the basic measurements available        (in this example a measure of OTD and one of TA) and, after        verifying whether they are sufficient (positive outcome of a        step 106) sends them to the SM module (step 108);    -   if the number of basic measurements obtained by the terminal is        not sufficient (negative outcome of the step 106), the SM module        conducts additional measurements (for instance of TOA), as        indicated in the step 110,    -   the SM module invokes the calculation of the mobile position        from the PCF function (step 112), and    -   the module SM processes the position of the mobile terminal        adding value-added information (for instance, advertising) and        sends the result to the terminal (step 114, followed by a final        step designated as 116).

As stated previously, the locating request can be initiated both by themobile terminal and by the SM module, which in turn can do so directlybased on a scheduling table or on request by an outside user or aservice provider connected through IP network.

In the first case, the mobile terminal directly conducts themeasurements of the power received by the base station, the related OTD,the value of TA for the serving base station and it may request from thenetwork the measurements of TOA (which the mobile terminal cannot takeautonomously) in addition to the geographic co-ordinates of the station,for instance by means of broadcast messages according to the RRLPprotocol (Radio Resource Link Protocol).

The PCF function on the mobile terminal estimates the position on thebasis of the information received following the described methodology.The information is displayed on the mobile terminal or sent to the SMmodule for additional processing in order to provide the client withvalue added services based on the geographic position of the mobile(e.g. yellow pages, tracking, etc.).

If, instead, it is the module SM that invokes the locating of the mobileterminal the function PCF on the network collects the necessary data,possibly requesting the mobile terminal to provide the OTD measurements(for instance by means of the RRLP protocol) and then calculates theposition of the mobile. The PCF function then returns to the mobileterminal, for instance via SMS, its position and/or other value-addedinformation that depend on the calculated position.

From the above, it is evident that the locating system according to theinvention can also operate in the presence of the combination ofmultiple errors in the space and time reference systems, such as thegeo-referencing errors of the radio base stations, the synchronisationerrors of the radio base stations and the errors in the measurement ofthe data to be used for locating purposes.

The system, according to the invention, can combine in wholly flexiblemanner a variable number of power measurements taken by the mobileterminal, of TA, of OTD and of TOA with the only limitation that atleast one measurement is indispensable.

The system, according to the invention, thus overcomes the limitationsof traditional positioning methods, based on the comparison between thereceived signals and those that should be received on maps prepared apriori (which, additionally, for practical reasons need to be tracedusing mathematical models which may introduce a difference from theactual situation) and which entail their continuous updating as themobile radio network evolves.

Lastly, the system, according to the invention, is not based on thesearch for the intersection of geometric curves, intersection which maynot exist due to space and time reference errors.

Naturally, without varying the principle of the invention, theimplementation details and the embodiments may vary from what isdescribed and illustrated herein, purely as examples, without therebydeparting from the scope of the invention.

1. Method for locating a mobile terminal (MS, MS2, . . . ) within a mobile communication network comprising at least one base station (BTS1, BTS2, . . . , BTSn), the method comprising the measurement of a set of physical dimensions that identify, according to respective functions, locating co-ordinates (x, y, z) of said mobile terminal, characterised in that it comprises the steps of: generating, starting from said set of physical dimensions and respective functions, a global locating error function (φ) which has a minimum for values of said locating co-ordinates (x, y, z) corresponding with the position occupied by said mobile terminal, seeking the minimum of said error function (φ) by varying at least one of said locating co-ordinates (x, y, z), and locating said mobile terminal in correspondence with the value of said at least one locating co-ordinate corresponding to said minimum.
 2. Method as claimed in claim 1, characterised in that said set of physical dimensions comprises at least a dimension selected within the group constituted by: signal power received by said mobile terminal starting from said at least one base station, Timing Advance (TA), Observed Time Differences (OTD), and Time of Arrival (TOA).
 3. Method as claimed in claim 1 or 2 characterised in that the measuring step comprises the step of performing measurements able to identify at least a value of position or distance with determined precision.
 4. Method as claimed in claim 1, 2 or 3, characterised in that said global error function is defined as the variance of the dimensions included in said set and a dimension whose value is zero.
 5. Method as claimed in claim 1, 2 or 3, characterised in that said global error is defined as the mean square error of the dimensions of said set.
 6. Method as claimed in any of the previous claims, characterised in that said global error function (φ) is obtained starting from a plurality of dimensions of said set.
 7. Method as claimed in claim 1, 2 or 3, characterised in that said set comprises one single dimension, so that said global error function (φ) is generated starting from the single dimension of said set.
 8. Method as claimed in any of the previous claims, characterised in that it comprises, to seek said minimum, the execution of an iterative process evaluating of said global error function for different values of said at least one location co-ordinate (x₀, y₀, z₀ . . . ; x_(n), y_(n), z_(n)) corresponding to successive different points of the space covered by said communication network.
 9. Method as claimed in claim 8, characterised in that it comprises the step of interrupting said iterative process when the absolute distance between two successive points is below a determined threshold value.
 10. Method as claimed in any of the previous claims, characterised in that it is applicable in a three-dimensional reference system.
 11. System for locating a mobile terminal (MS1, MS2, . . . ) within a mobile communication network comprising at least one base station (BTS1, BTS2, . . . BTSn), the system comprising at least a locating module (PCF) configured to measure a set of physical dimensions that identify according to respective functions location co-ordinates (x, y, z) of said mobile terminal, characterised in that said locating module (PCF) is configured to: generate, starting from said set of physical dimensions and respective functions, a global locating error function (φ) which allows a minimum for values of said locating co-ordinates (x, y, z) corresponding with the position occupied by said mobile terminal, seek the minimum of said error function (φ) varying at least one of said locating co-ordinates (x, y, z), and locate said mobile terminal in correspondence with the value of said at least one locating co-ordinate (x, y, z) corresponding to said minimum.
 12. System as claimed in claim 11, characterised in that said set of physical dimensions comprises at least one dimension selected in the group constituted by: signal power received by said mobile terminal starting from said at least one base station, Timing Advance (TA), Observed Time Differences (OTD), and Time of Arrival (TOA).
 13. System as claimed in claim 11 or claim 12, characterised by measuring devices able to obtain measurements to identify at least a position value of said mobile terminal or distance with a determined precision.
 14. System as claimed in claim 11, 12 or 13, characterised in that said global error function is defined as the variance of the dimensions included in said set and a dimension whose value is zero.
 15. System as claimed in claim 11, 12 or 13, characterised in that said global error function is defined as the mean square error of the dimensions of said set.
 16. System as claimed in claim 11, 12 or 13, characterised in that said locating module (PCF) is configured to obtain said global error function (φ) starting from a plurality of dimensions of said set.
 17. System as claimed in claim 11, 12 or 13, characterised in that said locating module (PCF) configured to obtain said global error function (φ) starting from said set comprises one single dimension, so that said global error function (φ) is generated starting from the single dimension of said set.
 18. System as claimed in any of the claims from 11 through 17, characterised in that to seek said minimum, said locating module (PCF) is configured to carry out an iterative process for evaluating said global error function for different values of said at least one locating co-ordinate (x₀, y₀, z₀; . . . ; x_(n), y_(n), z_(n)) corresponding to the successive different points of the space covered by said communication network.
 19. System as claimed in claim 18, characterised in that said locating module (PCF) is configured to interrupt said iterative process when the absolute distance between two successive points is below a determined threshold value.
 20. System as claimed in any of the claims from 11 to 19 characterised in that said error function (φ) is able to operate in a three-dimensional reference system.
 21. System as claimed in any of the claims from 11 to 20, characterised in that it further comprises a module (MGC) to allow the exchange of data between said mobile terminal and said at least one base station to identify at least one dimension of said set.
 22. Mobile terminal configured for use in a system as claimed in any of the claims from 11 to 21, characterised in that the terminal comprises at least part of said locating module (PCF) integrated in the mobile terminal itself.
 23. Software product able to be loaded directly into a memory of a digital computer associated with a mobile terminal (MS1, MS2, . . . ) as claimed in claim 22 and comprising portions of software code able to implement said at least part of said locating module (PCF) integrated in the mobile terminal itself when said software product is run on said digital computer.
 24. Communication network comprising at least a base station (BTS1, BTS2, . . . BTSn) and a plurality of mobile terminals (MS1, MS2, . . . ), the network comprising a locating system as claimed in any of the claims from 11 to
 21. 25. Communication network as claimed in claim 24, characterised in that it comprises an interface module (GW) for interfacing with an IP network, said interface module being configured in such a way as to allow the transfer of at least one between: an order to locate one of said mobile terminals starting from a source (U) connected to said IP network, and a delivery information generated by a source (U) connected to said IP network, directed to said mobile terminals (MS1, MS2, . . . ) and referred to the location of at least one of said mobile terminals. 